Many highly stressed fastener joints are used today, especially in the aerospace industry. Such fastener joints generally use a fastener with a head at one end, a bearing section which can be extended through the holes in the work pieces integral with the head and an engagement section at the opposite end of the bearing section which is designed to be engaged by a locking device such as a nut or collar to maintain the fastener in position. Several important criteria are usually followed in such fastener joints. One of these criteria is the maintenance of good bearing contact between the fastener and the work pieces. Another of these criteria is the keeping of the weight of the fastener joint at a minimum. Yet another of these criteria is that the resulting joint exhibit good fatigue life.
Generally, two types of fasteners are used in these fastener joints, one being a fastener having a substantially constant diameter bearing section that is placed through substantially constant diameter holes in the work pieces as is well known in the fastener art and the other being a fastener with a tapered bearing section that tapers outwardly from the engagement section toward the head which is placed in a correspondingly tapered hole through the work pieces as disclosed in U.S. Pat. Nos. 3,034,611 and 3,304,109. Both of these types of fasteners require a locking device such as a nut or collar to hold the fastener in place in the joint which has resulted in the joints formed therewith remaining relatively heavy thereby reducing the overall pay load of the structure in which they are used. Further joints made with these prior art fasteners required a significant number of installation steps in the formation of the fastener joint which maintained a relatively high installation cost.
The fatigue life of a fastener joint can be improved using a technique commonly known as coldworking wherein the hole is expanded to such an extent that the metal immediately surrounding the holes where the localized expansion occurs is stressed beyond its compressive yield point and after which the hole is permitted to return or rebound toward its original diameter to establish a compressive stress gradient immediately surrounding the hole or a technique commonly known as interference fit where the diameter of the bearing section of the fastener is greater than the initial nominal diameter of the holes in which the fastener is fitted. Because it is desirable to incorporate the coldworking and/or interference fit tooling and installation costs associated with the formation of a fastener joint, special fasteners such as those disclosed in U.S. Pat. Nos. 3,578,267 and 3,779,127 have been proposed. One of the problems in using these special fasteners is that the coldworking and/or interference fit tooling incorporated in the fastener remains an integral part of the completed fastener joint thus causing the joint to be relatively heavy. Such fasteners also still require the locking device such as a nut or collar, usually of special construction, thereon to retain the fastener in place which further keeps the joint relatively heavy.